There are two general types of concrete pipe making machines known as packerhead and vibrating core machines. Packerhead concrete pipe making machines have packerheads with rollers that are moved along an annular path and up into molds to form concrete pipes. An example of a packerhead concrete pipe making machine having a packerhead with elastic rollers is disclosed by Haddy in U.S. Pat. No. 4,690,631. The packerhead process of making concrete pipes is fast in operation and produces concrete pipes having smooth outside and inside finishes. Packerhead concrete pipe making machines have been proposed to simultaneously make two concrete pipes to increase pipe production. These machines require constant operator attention to ensure a supply of concrete above each packerhead necessary to produce two concrete pipes. An example of a packerhead concrete pipe making machine for simultaneously making two concrete pipes is disclosed by Christian in U.S. Pat. No. 4,118,165. Vibrating core concrete pipe making machines have generally cylindrical cores and vibrators mounted on the cores. The cores are moved up into molds to form concrete pipes. The vibrators vibrate the cores to consolidate and densify the concrete in the molds during the forming of the pipes. An example of a vibrating core concrete pipe making machine is shown by Fosse et al in U.S. Pat. No. 3,948,354.
The vibration process is a relatively slow method of making concrete pipes as compared to the packerhead process. However, the vibration process produces denser pipes and does not twist the reinforcing cages within the pipes. The outside finish of pipes made by the vibration process is not smooth due to a tendancy of air pockets to collect between the outside walls of the pipes and the molds.